Three-way normally closed hydraulic controllers are well known. These controllers are useful in many applications, including use in hydraulic control loops for oil field flow lines. They can be installed, for example, on hydraulic fluid lines supplying hydraulic pressure to flow line safety valves. One such controller is sold by Sigma Enterprises, Inc., of Dallas, Tex., under Assembly No. 20HM24. The 20HM24 controller comprises a supply port, a valve port and an exhaust port, and is adapted by means of an internal valve to selectively provide fluid communication between the valve port and either the supply port or the exhaust port.
The supply port is ordinarily connected to a source of pressurized hydraulic fluid. The valve port is typically connected to a flow line valve such as a flow line safety valve. When the valve port and supply port are in fluid communication, hydraulic pressure is exerted on the flow line valve, causing the flow line to remain open. When the valve port and exhaust port are in fluid communication, the hydraulic pressure from the supply source is blocked, and the pressure on the flow line valve is bled off through the exhaust valve of the controller, causing the flow line valve to close. For safety reasons, the controller is designed so that the valve port and supply port are not in fluid communication unless another external force is exerted against the internal valve of the controller. This feature give rise to the term "normally closed." Under standard operating conditions, pressurized fluid also communicates through pilot lines between remote sensors and the pilot port of the controller. When no alarming condition exists that is sufficient to trip the remote sensors, the fluid pressure exerted through the pilot port of the controller acts against the "normally closed" internal valve to provide fluid communication between the valve port and the supply port. If pilot pressure subsequently fails, the internal valve automatically reverts to its "normally closed" position.
In working with such control systems, it is sometimes desirable to maintain fluid communication between the valve and supply ports in the controller even in the absence of pilot pressure while permitting operation of the continued primary flow line. This situation might exist, for example, if it were necessary to repair or replace a portion of the pilot pressure system. Also, it is sometimes desirable to operate the controller in a hydraulic trap system where no pilot pressure is available. A manually operated override mechanism is employed in the 20HM24 to permit the operator to hold the internal valve in its "open" position.
However, in spite of the capabilities of the prior art devices, certain disadvantages have also been encountered during their use and operation. One disadvantage occurs when the conventional controller is subjected to elevated temperatures while the override mechanism is engaged. In such an instance it might be impossible to manually disengage the override mechanism, and thereby block and bleed the fluid pressure to the flow line valve. Another disadvantage of conventional controllers relates to the difficulty experienced by operators in determining by a visual inspection of the controller whether the internal valve is in the "in service" or "block and bleed" position, and if in service, whether or not the manual override is engaged.
A three-way normally closed hydraulic controller is therefore needed that will automatically revert to the "block and bleed" configuration when subjected to elevated temperatures while the manual override is engaged. A three-way normally closed hydraulic controller is also needed that will permit the operator to tell by visual inspection of the controller whether the internal valve is in the "in service" or "block and bleed" position and whether or not the manual override is engaged.